Aviation fuel with a renewable oxygenate

ABSTRACT

Described are preferred compositions for a motor fuel. Such motor fuels may be particularly well suited for use in the motor of an aircraft. In particular, compositions of the present disclosure may comprise 50-75 wt % isooctane/alkylates, 20-40 wt % ETBE, 0-3 wt % isobutane, and 0-5 wt % aromatics. The present disclosure describes a full spectrum of unleaded fuels with various motor octane (MON) values.

REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 62/024,028, filed Jul. 14, 2014, the contents ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to lead-free piston engine fuels (unleadedavgas) comprising aliphatic hydrocarbon components, typically includinglower boiling C₄ to C₁₀ alkanes, alkenes, cycloalkanes and arenes foundin gasoline, plus the use of oxygen-based heteroatomic compounds,particularly ETBE, blended together to produce unique avgas formulationswith a 98 or higher motor octane number that offers excellent engine andoperational performance for aviation purposes. These unique fuels areshown to have a) excellent piston-engine combustion and exhaustcharacteristics, b) lower environmental toxicity compared to aromaticamines or metals used as octane boosters, and c) a selectively highdegree of fuel compatibility with materials used in aircraft fuelsystems.

DESCRIPTION OF THE PRIOR ART

Motor fuels are used in a variety of systems. In the broadest sense, amotor fuel is one which is used in piston or turbine engines. Thepresent invention is directed to fuels for piston engines useful inground vehicles and/or aircraft. Typically, ground vehicles can userelatively lower octane fuels, while aircraft require higher octanefuels. A basic determinant as to the choice of fuels is the octanerating of the fuel compared to the compression of the engine. Forexample, higher compression engines generally require higher octanefuels.

A particular aspect of the present invention is to provide formulationswhich are useful as piston engine fuels, and are particularly suited foruse as aviation gasoline.

Aviation gasoline, or avgas, has a number of special requirements ascompared to ground vehicle gasoline. Aviation gasoline (called “avgas”)is an aviation fuel used in spark-ignited (reciprocating) piston enginesto propel aircraft. Avgas is distinguished from mogas (motor gasoline),which is the everyday gasoline used in motor vehicles and some lightaircraft.

Most grades of avgas have historically contained tetraethyl lead (TEL),a toxic substance used to prevent engine knocking (detonation). Thisinvention produces an unleaded grade of avgas with fuel properties thatsatisfy the appropriate combustion and anti-knocking requirements(detonation suppression), volatility (vapor pressure), and relatedcriteria for piston engine aircraft as defined by ASTM D910 for 100LL(leaded avgas), but with a minimum 98 motor octane number. The inventivefuels allow a range of piston engine aircraft, includinghigh-compression piston engines, to perform effectively to manufacturerrequirements.

Aviation gasoline must meet the power demands for aircraft engines. Themotor octane number, or MON, is a standard measure of the performance ofan aviation fuel. The higher the MON, the more compression the fuel canwithstand before detonating. In broad terms, fuels with a higher motoroctane rating are most useful in high-compression engines that generallyhave higher performance.

The MON is a measure of how the fuel behaves when under load (stress).ASTM test method 2700 describes MON testing using a test engine with apreheated fuel mixture, 900 rpm engine speed, and variable ignitiontiming to stress the fuel's knock resistance. The MON of the aviationgasoline fuel can be used as a guide to the amount of knock-limitingpower that may be obtained in a full-scale engine undertake-off, climband cruise conditions.

Another particular issue with avgas is its ability to start reliablyunder a wide range of altitude and climate conditions. Avgas needs tohave a lower and more uniform vapor pressure than automotive gasoline soit remains in the liquid state despite the reduced atmospheric pressureat high altitude, thus preventing vapor lock. The ability of an aviationgasoline to satisfy this requirement may be assessed based on the ReidVapor Pressure (RVP). A typical requirement for avgas is that it have anRVP of 38-49 kPa at 37.8° C., as determined in accordance with ASTMD5191.

Avgas must also be highly insoluble in water. Water dissolved inaviation fuels can cause serious problems, particularly at altitude. Asthe temperature lowers, the dissolved water becomes free water. Thisthen poses a problem if ice crystals form, clogging filters and othersmall orifices, which can result in engine failure.

Accordingly, ethanol and alcohol components are generally not used inaviation fuels due to their tendency to be water soluble, and somecompounds are highly corrosive to fuel system components.

These fuels may optionally include other components or additives,particularly to modify or enhance characteristics such as octane rating,vapor pressure, viscosity, anti-icing, anti-static, oxidation stability,anti-corrosion, boiling point, engine cold start, exhaust smoke andengine deposits.

Aviation fuels are a product of blending many possible hydrocarboncomponents to very specific formulations to create a combustible fuelthat is tailored for an aviation specific use. For example, turbineengines used on most commercial jets worldwide utilize jet fuelsspecifically design for their combustion characteristics usinghydrocarbons with longer-chain molecules with carbons typically rangingbetween C₈ to C₁₆. These fuels typically have a high flash point (lessflammable) which makes them safe for handling in a wide range ofcommercial uses. Piston engines used in general aviation require fuelsmade from lighter hydrocarbons (typically ranging from C₄ to C₁₀ carbonmolecules) similar to gasolines used in automobiles, but with muchhigher octane requirements and somewhat lower vapor pressurerequirements. For many decades the combustion characteristics of avgasused by piston engine aircraft has required tetraethyl-lead as a keycomponent to the fuel to achieve the highest levels of motor octanenumber—thereby helping to reduce the likelihood of engine knocking. Inrecent years, the combination of public health hazards and environmentalregulations has triggered an effort across the global aviation industryto remove all lead compounds from avgas.

The alternatives for blending and producing a lead-free aviationgasoline which meets the performance requirements for all varieties ofpiston engine aircraft are complex even for those schooled in the art ofaviation gasolines. Aviation fuels used in piston engine aircraft mustmeet all minimum performance criteria as defined by various fuelspecifications managed by ASTM International and overseen by across-industry forum of experts. The fuel must also meet minimum fueloperating requirements as defined by Federal Aviation Administration(FAA) and other federal, state and local regulators. Specifically theavgas must meet the minimum motor octane number to assure appropriateknock suppression under a range of engine performance requirements, theappropriate range for vapor pressure and all related matters impactingcombustion, volatility, composition, fluidity, anti-corrosion, oxidationstability, environmental toxicology and material compatibility.

Compounds that have been found to enhance the motor octane rating ofavgas for piston aircraft, as studied by those schooled in the art ofaviation gasolines, include fuels with high concentrations of aromatichydrocarbons (particularly methylbenzene, dimethylbenzene or1,3,5-trimethylbenzene), or fuels blended with various aromatic amines(particularly aniline or meta-toluidine), oxygenates (e.g. MTBE, ETBEand Ethanol) and/or certain metals (particularly tetraethyl lead). Thisinvention focuses on the use of base aliphatic compounds using specificC₄ to C₁₀ hydrocarbons, blended in the absence of nitrogen-basedaromatic amines and in the absence of metals, but with the addition ofvery specific oxygen-based heteroatomic molecules (oxygenates) toachieve lead-free fuels that meet the appropriate ASTM specificationsfor aviation gasoline with a minimum 98 motor octane number. Furthermorethe fuel is shown to be safe, low in toxicity, excellent combustioncharacteristics and fully compatible with materials used in aircraftfuel systems and the related supply chain.

U.S. Pat. No. 5,851,241 describes an unleaded aviation fuel comprised ofbase alkylate combined with an alkyl-tertiary-butyl-ether (typicallyMTBE or ETBE) in combination with up to 10% of an aromatic amine (e.g.aniline, m-toluidine, etc.); some derivative formulations also includethe use manganese as an octane booster. Since MTBE and manganese hasbeen largely banned in transportation fuels across many states in the USover the past 10 years, these formulations are not commercially viablein the marketplace. Furthermore, the use of high concentrations ofaromatic amines brings concerns of environmental toxicity into the fuelformulations further challenging their acceptance as a fuel in themarketplace.

U.S. Pat. No. 6,238,446 describes various lead-free aviation fuels witha minimum 100 MON based upon a blend combination of base alkylate with4% to 10% MTBE (or ETBE, or MTAE) plus the addition of 0.2-0.6 grams ofmanganese per gallon. This application fails to look at the high wearand tear impact of metals on the piston engine, or the impact theseethers like MTBE which are banned in the US marketplace. These factorsmake this invention impractical and commercially undesirable foraviation use.

US Patent Application No. 2008/0244963 A1 describes an unleaded fuelblended from a base aviation gasoline, with a minimum 100 MON, whichcontains various combination of alkylates, ethers, ether alcohols,anhydrides, aromatic ethers and ketones. Many of these fuel componentshave environmental toxicity issues that make this invention impracticaland commercially undesirable for aviation use.

The Federal Aviation Administration (FAA) testing over a 10-year period,from 1990 to 2000, evaluated ETBE as a possible component for unleadedaviation gasoline. All ETBE-based formulations tested by the FAA programrequired the use of aromatics amines (i.e. m-toluidine) ortert-butyl-benzene to effectively boost the octane performance of thefuel for adequate piston engine anti-detonation performance.

Many other attempts have been made at devising a lead-free high-octaneaviation gasoline starting from a hydrocarbon-based aviation fuel, someby combining lower boiling alkylates and aromatics up to 80% to increasethe octane, as well as 5-15% of additional C4-C5 compounds to adjust thevapor pressure to aviation gasoline standards. See, for example, U.S.Pat. Nos. 8,741,126, 7,416,568, 8,324,437, 8,049,048, and 8,686,202.Unlike these 5 hydrocarbon-specific fuel examples, the use of oxygenatescombined with either MMT and/or aromatic amines into the base aviationfuel, as described in the prior art above, has resulted in a heightenedconcern industry wide to understand a broader view of the operationalrisks of these fuels on the aviation industry. That is what thisselective research on ETBE and the associated invention herein hasfocused upon.

In light of this background, there remains a need for additional and/orimproved fuel compositions.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides for an improved fuelcomprising ETBE and selected aliphatic hydrocarbons. For example,compositions of the present invention with a high motor octane number(MON) of 98 or above and suitable boiling point characteristics(impacting fuel stability, cold starting features, exhaustcharacteristics, etc.) may be useful as aviation fuel for many types ofaircraft engines including high-performance engines and also legacyaircraft.

In another aspect, the present invention provides for an improved fuelthat contains a minimal amount of lead compounds to achieve its optimaldetonation suppression characteristics. For example, certaincompositions of the present invention do not include the use of anytetraethyl lead or any ethylene dibromide to scavenge for the lead inthe aircraft fuel system.

In still another aspect, the present invention provides for an improvedfuel that meets or exceeds one or more requirements of ASTM D910 and/orASTM D7719 and/or ASTM D7547.

Additional embodiments of the invention, as well as features andadvantages thereof, will be apparent from the descriptions herein.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to certain embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications, and such further applications of the principles of theinvention as described herein being contemplated as would normally occurto one skilled in the art to which the invention relates.

ETBE is an aliphatic ether derived from the processing of ethanol(notably from bio-sources) and isobutylene. The ETBE molecular structurecontains oxygen, hence it is called an oxygenate. ETBE has a positiveimpact on octane in the combustion of a piston engine. However, theenergy density is about 5-8% less per gallon—resulting in a loss ofaircraft range. This is reflected in a lower net heat of combustionmeasured by ASTM fuel standards. The oxygen in ETBE produces a favorablecombustion affect, which tends to make a more complete combustion (thusemitting fewer unburned hydrocarbons in the exhaust). ETBE has favorablematerial compatibility features in that it is not aggressive in actingagainst the materials in an aircraft fuel system. ETBE has a watersolubility of 1.2 g/100 g which can contribute to combustion issues incold weather. Also, the boiling point of 71° C. results in somedifficulty starting in extreme fuel with ETBE in cold weathersituations. This is observed at the 10% boiling point (at 85° C., max)on the ASTM distillation curve test.

The present invention provides unleaded, piston engine fuels preferablycomprising a mixture of select aliphatic hydrocarbons blended with ETBE.The aliphatic hydrocarbons may include alkanes, alkenes, alkynes,cycloalkanes and alkadienes. In preferred embodiments, the aliphatichydrocarbons comprise lower boiling C₄ to C₁₀ alkanes, alkenes andcycloalkanes, but largely excluding arenes found in gasoline. Theresulting fuel formulations are characterized by an array of desirableproperties making them suitable for piston engines.

In certain aspects, the fuels comprise an alkylate product consisting ofa variety of hydrocarbons. In refining, the alkylation processtransforms low molecular-weight alkenes and iso-paraffin molecules intoa product referred to as an “alkylate”, which includes a mixture ofhigh-octane, isoparaffins. As used herein, the term “alkylate” refers tothe alkylate product available from a refinery, and also generally toany mixture including C4 to C10 non-aromatic hydrocarbons. Whether fromthe alkylate product of the refineries, or in more purified form, theinclusion of these high volatility/low boiling point componentscontributes to achieving a desired Reid Vapor Pressure (RVP) range.

In one aspect, the alkylate component comprises alkanes. In particular,it has been found that the C4-C10 alkanes, and more preferably branchedalkanes, provide especially desirable properties for the inventive fuelformulations. Isooctane is particularly preferred in order to achieve abalance of desirable fuel properties.

Aspects of the present invention relate to compositions of fuel. Moreparticularly, aspects of the present invention may be particularlyapplicable to fuel compositions used for aircraft, often called aviationgasoline or avgas. ASTM specification D7719 describes a fuelspecification for high octane aviation fuel, and is hereby incorporatedby reference in its entirety. ASTM D7719 also makes reference todocuments, for example but not limited to other ASTM specifications, andthese references are hereby incorporated by reference in their entirety.ASTM specification D7547 describes a fuel specification for unleadedaviation fuel. ASTM D7547 is hereby incorporated by reference in itsentirety. ASTM D7547 also makes reference to documents, for example butnot limited to other ASTM specifications and these references are herebyincorporated by reference in their entirety. ASTM specification D7592describes a fuel specification for unleaded aviation fuel. ASTM D7592 ishereby incorporated by reference in its entirety. ASTM D7592 also makesreference to documents, for example but not limited to other ASTMspecifications, and these references are hereby incorporated byreference in their entirety. ASTM specification D910 entitled “StandardSpecification for Aviation Gasolines” describes several characteristicsthat an aviation gasoline may meet, and is hereby incorporated byreference in its entirety. ASTM D910 also makes reference to documents,for example but not limited to other ASTM specifications, and thesereferences are also hereby incorporated by reference.

TABLE 1 ASTM D7719 (UL 102) provides as follows: D4809 Net Heat ofCombustion, MJ/kg Octane Rating 41.5, min D2700 Knock value, leanmixture Motor Octane 102.2, min Number D2622 Sulfur, mass % 0.005, maxD5059 Tetraethyl lead, mL g Pb/L 0.013, max D5191 Vapor pressure, 38°C., kPa 38-49 D1298 Density at 15° C., kg/m3 790-825 D86 DistillationFuel Evaporated 10, volume % at ° C. 75, max 40, volume % at ° C. 75,min 50, volume % at ° C. 165, max 90, volume % at ° C. 165, max Finalboiling point, ° C. 180, max Sum of 10% + 50% evaporated 135, minRecovery, volume % 97, min Residue, volume % 1.5, max Loss, volume %1.5, max D2386 Freezing Point, ° C. −58, max D130 Corrosion, copperstrip, 2 h @ 100° C. No. 1, max D873 Oxidation stability (5 h aging)Potential 6, max gum total, mg/100 mL D1094 Water reaction, Volumechange, mL ±2, max D2624 Electrical conductivity, 19.9° C., pS/m 450,max

It has been found that the present fuel formulations have a minimum 98motor octane number (MON) that satisfactorily supports anti-detonationtests in a full-scale engine test. Compositions of the present inventionhave a MON of at least 98 depending on the actual blend of componentsused. The fuel formulations have an RVP of 38 to 49 kPa at 37.8° C.

The unleaded fuel in the invention, also called “UL100R” or “100R” inTable 2, compares favorably to ASTM D910 Grade 100LL and ASTM D6227Grade UL87 below with regard to performance properties in Table 1. Forexample, UL100R has a net heat of combustion minimum that is 2.7 MJ/kglower than that for 100LL, and when converted to a volume basis (MJ/L),the net heat of combustion is actually 5-8% lower than 100LL. Researchhas indicated that the presence of an oxygenate in the fuel results in amore complete combustion, which offsets some of the effect of thereduced net heat of combustion. The impact of the more completecombustion, on a per gallon basis, allows the range of flight of theaircraft to be equivalent to that of 100LL while the exhaust emissionsare far cleaner with UL100R (i.e., no lead exhaust, and lower unburnedhydrocarbons in the exhaust due to the presence of oxygen at the time ofcombustion). While UL100R has a minimum MON of 98, the presence of anoxygenate results in improved combustion performance, which providessome knock resistance enhancement compared to a non-oxygenated fuel ofequivalent MON.

UL100R is an unleaded fuel that allows for up to 0.013 gPb/L maximum incase of accidental contamination between the refinery and the FBO,whereas 100LL is a leaded fuel that contains up to 0.56 gPb/L. UL100R,being an unleaded fuel, will have zero lead precipitate. UL100R is anoxygenated fuel, containing up to 40% (m/m) ethyl tert-butyl ether(ETBE), which is preferably made from bio-ethanol and isobutylene;therefore, with 40% ETBE in the fuel, any ETBE derived from corn ethanolis calculated as 18% sourced from renewable feedstocks. It will beappreciated, however, that the present invention is not restricted tothe use of ETBE obtained from any particular source. ETBE alone has beenendorsed by the FAA as a viable fuel component despite market concernsabout continued multi-state bans of MTBE.

TABLE 2 Comparison of UL100R to ASTM D910 (Grade 100LL) and ASTM D6227(Grade UL87) ASTM Test Leaded Unleaded Unleaded Method ASTM RequirementsASTM D910 ASTM D6227 UL100R Grade 100LL, UL87 UL100 R Avgas COMBUSTIOND4809 Net Heat of Combustion, MJ/kg 43.5, min 40.8, min 40.8, min OctaneRating D2700 Knock value, lean mixture Motor Octane Number 99.6, min87.0, min 98, min Aviation Lean Rating 100, min D2699 Research OctaneNumber D909 Knock value, rich mixture Octane Number Performance number130, min COMPOSITION D2622 Sulfur, mass % 0.05, max 0.07, max 0.005, maxD5059 Tetraethyl lead, mL TEL/L 0.53, max g Pb/L 0.56, max 0.013, maxD2392 Color blue Dye content Blue dye, mg/L 2.7, max Yellow dye, mg/Lnone 2.8, max Red dye, mg/L none Orange dye, mg/L none VOLATILITY D5191Vapor pressure, 38° C., kPa 38-49 38-62 38-49 D1298 Density at 15° C.,kg/m³ Report Report 730 max D86 Distillation Initial Boiling Point, ° C.Report Report Fuel Evaporated 10, volume % at ° C. 75, max 70, max 85,max 40, volume % at ° C. 75, min 75, min 50, volume % at ° C. 105, max66-121 105, max 90, volume % at ° C. 135, max 190 135, max Final boilingpoint, ° C. 170, max 225 170, max Sum of 10% + 50% evaporated 135, min135, max Recovery, volume % 97, min 95, min 97, min Residue, volume %1.5, max 2.0, max 1.5, max Loss, volume % 1.5, max 3.0, max 1.5, maxDriveability Index Observed Condition FLUIDITY D2386 Freezing Point, °C. −58, max −58, max −58, max CORROSION D130 Corrosion, copper strip, 2h @ No. 1, max No. 1, max No. 1, max CONTAMINANTS D873 Oxidationstability (5 h aging) Lead Precipitate, mg/100 mL 3, max Potential gum,mg/100 mL 6, max 6, max 6, max D1094 Water reaction Interface ratingSeparation rating Volume change, mL ±2, max ±2, max OTHER D2624Electrical conductivity, 19.9° C., 450, max 450, max

The UL100R fuel is a 98+ octane unleaded aviation gasoline with up to18% renewable content that meets most of the primary ASTM D910parameters and offers the cleanest exhaust emissions. The base fuelcontains no intentional aromatic hydrocarbons (e.g., toluene, xylene,and trimethylbenzenes) as these can increase the density of the fuel andthereby change the weight distribution of the aircraft. Certainembodiments do however allow up to 5% aromatics to improve octaneperformance. The preferred embodiment of UL100R without aromatics has adensity identical to 100LL. The lower net heat of combustion may resultin up to 5-8% less range in the aircraft; tests have indicated, however,that UL100R burns more completely than other unleaded fuel compositions,which may offset some of this loss of range.

UL100R has low overall toxicity due to the usage of gasoline componentscoupled with ETBE, which are not classified under OSHA's Acute Toxicityrating scale. The ETBE used in UL100R must satisfy the minimum qualityrequirements, as specified in ASTM D7618, Standard Specification forEthyl Tertiary-Butyl Ether (ETBE) for Blending with AviationSpark-Ignition Engine Fuel. In some embodiments, the fuel may alsocontain an additive of up to 250 ppm of ferrocene, a non-toxiciron-based octane booster. Research has indicated that ETBE alone, or incombination with certain alkylates, can in fact meet the anti-knockdetonation requirements of piston engines without the use of octaneboosters; however, with the addition of up to 250 ppm of ferrocene, theUL100R fuel can meet or exceed the minimum octane levels of 100LL.

The toxicity of ETBE was compared to other common components in aviationgasoline. Here below is a brief recap:

TABLE 3 Component LD₅₀ (rat, oral) OSHA Hazards Mesitylene 5,000 mg/kgIrritant ETBE 5,000 mg/kg Irritant Toluene 5,000 mg/kg Irritant,Teratogen, Reproductive hazard Benzene 2,990 mg/kg Carcinogen, Mutagen,Irritant Cumidine   757 mg/kg Irritant. Causes respiratory tractirritation. Causes eye and skin irritation. Can form methemoglobin, maycause cyanosis. May cause central nervous system depression. m-Toluidine  450 mg/kg Toxic. Causes cyanosis. Harmful or fatal if inhaled,swallowed, or absorbed through skin. May be irritating to skin, eyes andmucous membranes. Target organs: Bladder; kidneys; blood; liver. Aniline  250 mg/kg Carcinogen, toxic if swallowed, toxic in contact with skin,causes skin irritation, causes serious eye damage, fatal if inhaled,suspected of causing genetic defects. Dibromoethane   55 mg/kgCarcinogen, toxic by inhalation, toxic by skin absorption. TEL   14mg/kg Carcinogen, toxic by inhalation, highly toxic by ingestion, highlytoxic by skin absorption, teratogen. Source: SDS data from third-partycompliance reports

This summary highlights the relative acute toxicity based on public datausing LD₅₀ as an internationally accepted baseline. In addition, chroniceffects from long term exposure and other effects like carcinogenicity,mutagenicity, and teratogenicity have to be considered for the objectiveevaluation of the fuel.

Another key factor is the relative concentration of potentially toxiccomponents in a particular fuel formulation, e.g. certain aromaticamines may require 60 to 250 times the concentration level in ahigh-octane unleaded aviation fuel vs. TEL found in 100LL. See, Albuzat,T., Understanding the Merits of 1,3,5-Trimethylbenzene. CoordinatingResearch Council Aviation Meetings, Apr. 28, 2014, p. 6. For thisreason, UL100R is tailored as a special non-toxic formulation withchemical components that exceed the bounds of OSHA standards for acutetoxicity.

Pre-combustion: UL100R fuel is a flammable hydrocarbon liquid. Itevaporates more quickly than 100LL. If exposed to the skin, it is onlyan irritant. With regard to ecological risks, UL100R is expected topersist in soil and water, and it degrades more slowly in the absence ofoxygen, which is why proper industry-wide control of avgas tankage(leaks) is vital for acceptance of UL100R.

Post-combustion: UL100R is a clean-burning fuel with a much morecomplete combustion than 100LL due to the presence of oxygenates in thefuel. 100LL is known to emit rather white smoke containing toxic leadcompounds like lead oxides and lead bromide. These lead emissions arenot visible to the general population.

The composition of UL100R, being an oxygenated fuel, has pre- andpost-combustion occupational exposure limits similar to those ofautomotive gasoline, which typically range from 25 ppm-300 ppm [TWA: 8hours OSHA].

A basic component of the inventive fuel formulations is ETBE. The ETBEis used in an amount of about 20 to about 40 wt %, based on the overallweight of the formulation. In addition, a hydrocarbon component isincluded in an amount of about 60 to about 80 wt %. The hydrocarboncomponent is a constituent selected from the group consisting of C4-C10aliphatic hydrocarbons, alkylates and alkanes. In some embodiments aportion of the hydrocarbon component is replaced with one or more othercomponents selected from the group consisting of C6-C10 aromatichydrocarbons, isobutane, ferrocene and cumidine. Preferably when botharomatic hydrocarbons and cumidine are present in the formulations, theaggregate of the aromatic hydrocarbons and of the cumidine is no greaterthan 5 wt %.

Cumidine refers to three isomeric liquid bases (C₃H₇C₆H₄NH₂) derivedfrom cumene. It has been discovered that cumidine has unique propertiesfor an aromatic amine related to high octane aviation gasoline. In thepresent invention, the isomer 4-isopropylaniline is preferably used.

In one embodiment, the fuel composition UL100R results in theperformance properties specified herein. In the following formulas, theterm “alkylates” is intended to also include separately C4-C10 aliphatichydrocarbons. This fuel contains the following range of components byweight:

-   -   (Iso) butane: 0-3%    -   (bio-) ETBE: 20-40%    -   Isooctane/Alkylates: 50-75%    -   Aromatics Content: 0-5%        In a preferred embodiment, the formulation comprises, or        consists essentially of, 52-80 wt % alkylates (or aliphatic        hydrocarbons), 20-40 wt % ETBE, 0-5 wt % C6-C12 aromatic        hydrocarbons, up to 3 wt % isobutane, and up to about 250 ppm        ferrocene.

In a preferred embodiment, the formulation comprises, or consistsessentially of, 58-78 wt % alkylates (or aliphatic hydrocarbons), 20-40wt % ETBE, 2 wt % isobutane, and about 250 ppm ferrocene. In a preferredembodiment, the fuel formulation comprises, consists essentially of, orconsists of 58 wt % isooctane, 40 wt % ETBE, and 2 wt % isobutane, andhas a MON of about 100.

Another fuel composition of UL100R results in the performance propertiesspecified in the table above. The fuel contains the following range ofcomponents, by mass:

-   -   (Iso) butane: 0-3%    -   (bio-) ETBE: 20-40%    -   Isooctane/Alkylates: 50-75%    -   Aromatics Content: 0-5%    -   Up to 250 ppm of ferrocene

For example, the fuel formulation comprises, consists essentially of, orconsists of 58 wt % isooctane, 40 wt % ETBE, 2 wt % isobutane, and 250ppm of ferrocene.

In another embodiment, the fuel composition and contains the followingrange of components, by mass:

-   -   (Iso) butane: 0-3%    -   (bio-) ETBE: 20-40%    -   Isooctane/Alkylates: 50-75%    -   Cumidine: 0-5%

In another example, the fuel formulation comprises, consists essentiallyof, or consists of 53 wt % isooctane, 40 wt % ETBE, 5% cumidine, and 2wt % isobutane.

Due to the strict technical parameters outlined in D910, the UL100R fuelcomposition is tightly constrained by performance metrics, for exampleRVP, MON, and distillation curve. UL100R meets most of the performancecharacteristics of the ASTM International D910 aviation gasolinespecification, as outlined below.

Combustion performance of UL100R, as measured by knock resistance duringcombustion, is as good as or better than that of 100LL. UL100 Renewablehas a lower net heat of combustion by mass (40.8 MJ/kg) than 100LL (43.5MJ/kg) because of the oxygenate content. Due to the similar density, theheat of combustion on a volumetric basis is actually 5-8% less than100LL, however the combustion efficiency offsets this loss.

Fluidity is a critical operating parameter for flight safety. Thefluidity of UL100R is consistent with 100LL, with a freezing pointmaximum of −58° C. The physical properties of the components in UL100Rwork together to meet the rigorous requirement necessary to ensure thatfuel will continue to flow in a liquid state during high-altitudeoperations.

Volatility of the fuel is another critical operating parameter forreliability and flight safety. UL100R meets the traditional aviationgasoline standard of 38-49 kPa due to the presence of not more than 3%isobutane. Our tests reveal that fuels with (iso)butane concentrationshigher than 3% will exceed the maximum vapor pressure limit andexperience loss>1.5%. Fuels that are too volatile can experience vaporlock under normal operating conditions, or causing the engine not tostart on the ground, or not restarting in an emergency situation ataltitude.

Stability of UL100R is high due to the stable nature of the components.UL100R meets the strict oxidation stability requirements of ASTM D910for 100LL but without the risk of lead precipitate, as it is an unleadedfuel. Due to the fact that UL100R is composed of all hydrocarboncomponents, it is water-insoluble.

Corrosion testing has shown that UL100R meets the strict D910 standardfor accelerated soak testing of a copper strip.

Using a maximum quantity of 40% (m/m) bio-ETBE, UL100 Renewable achievesa Motor Octane Number of 98, which offers sufficient detonationprotection with no aromatic content needed to enhance anti-knockperformance. Due to the presence of oxygenates and iron in thisformulation, it is anticipated that equivalent anti-knock performancewill be achieved with a MON of 98+.

These formulations serve the entire piston-engine aviation fleet. Thisconsiders the needs of aircraft across the following range:

TABLE 4 Minimum Fuel Grade Distribution Percent of 189,415 Aircraft MinFuel Grade Number of Aircraft (Rounded) Minimum Grade 100LL 82,03443.3%  Minimum Grade 80 69,397 36.6%  Other Fuel 17,508 9.2% MinimumGrade 91 13,387 7.1% Unknown, etc. 5,302 2.8% Unleaded 91/96 825 0.4% 87octane 802 0.4% Jet A 147 0.1% Minimum Grade 90 13 0.0068%   Source:Crown Consulting, Inc. - General Aviation Piston Engine Fleet Assessmentfor Octane Requirement

The fuels meet the varied needs of the engines that make up thepiston-engine aviation fleet, including carbureted, fuel-injected,naturally-aspirated, turbocharged, supercharged, intercooled,low-compression, high-compression, horizontally-opposed, radial, in-lineengines and V-configuration engines.

Preliminary testing in an engine test cell has indicated that UL100Renewable achieved cold start at −20° C., and the engine performanceresults were “positive”. See FIGS. 1 and 2. The fuels demonstrated thefollowing properties. Cold Start: Both fuels started below −20° C. EGT:UL100 Renewable ran on average 25-50° C. hotter. CHT: UL100 Renewableran on average 5-15° C. hotter. Fuel Consumption: ran equivalently forboth fuels. This test experienced occasional misfires on 100LL, whichreduced the EGT and CHT. Also note: in an unmodified engine, the ExhaustGas Temperature is higher with UL100R because the oxygen in the fuelresults in a leaner burn (i.e. a higher air-to-fuel ratio) thus a hottertemperature. Carburetor adjustments can easily compensate for the thisaffect.

L100R fuel is compatible with all existing aircraft materials, bothmetallic and nonmetallic. UL100R is compatible with the existing fleetand related supply chain infrastructure. Related to seal swell, certainengine manufacturers may advise that all aircraft and fieldinfrastructure equipment that rely on Neoprene, Buna, or Vinyl Rubbermaterials be transitioned to Viton or Teflon materials (in most casesthese parts are cheaper and have a longer service life). Based on testresults, there is no immediate transition required for using UL100R,although this may be a prudent course of action for any aircraft beingoverhauled. Other alternatives that include certain aromatic aminecomponents, because of their more aggressive nature toward theaforementioned materials and their tendency to reduce tensile strength,would require an immediate, pre-emptive change-out of Buna, VinylRubber, and Neoprene components to satisfactory materials before thosealternative fuels could see active service in the fleet or distributioninfrastructure.

All 102-octane unleaded avgas candidates will face long-term materialcompatibility challenges related to Buna, Vinyl Rubber, and Neoprene.Testing on UL100R indicates that change-out of such materials may not benecessary until normal scheduled maintenance intervals, i.e., change-outis not a prerequisite for using UL100R.

The inventive fuels may “comprise” the described formulations, in whichother components may be included. However, in a preferred embodiment,the inventive fuels “consist of” the described formulations, in which noother components are present. In addition, the inventive fuels may“consist essentially of” the formulations, in which case other fuelexcipients may be included. As used herein, the term “fuel excipients”refers to materials which afford improved performance when used withfuels, but which do not directly participate in the combustionreactions. Fuel excipients thus may include, for example, antioxidants,etc.

The formulations are also useful for combining with other fuelcomponents to form blends that are useful as motor fuels, including asaviation gasoline. As used herein, the term “fuel components” refers tomaterials which are themselves combustible and have varying motor octaneratings and are included primarily to provide improved combustioncharacteristics of the blend. In preferred embodiments, such fuelcomponents are present in the blend at less than 5 wt %, and morepreferably less than 1 wt %.

Blending of the formulations herein can be performed in any suitableorder. No language in the specification should be construed asindicating any non-claimed element as essential to the practice of theinvention.

Most grades of avgas have historically contained tetraethyl lead (TEL),a toxic substance used to prevent engine knocking (detonation). Thisinvention produces an unleaded grade of avgas with fuel properties thatmeet minimum power rating (motor octane number), appropriate combustionanti-knocking (detonation suppression), volatility (vapor pressure), andrelated criteria. The inventive fuels allow a range of piston engineaircraft, including those with high-compression engines, to performeffectively to manufacturer requirements. It is necessary that avgasprovide sufficient power under varying conditions, including take-offand climb as well as at cruise.

Tetraethyl lead, abbreviated TEL, is an organolead compound with theformula (CH₃CH₂)₄Pb. It has been mixed with gasoline since the 1920's asan inexpensive octane booster which allowed engine compression to beraised substantially, which in turn increased vehicle performance andfuel economy. Over the years, certain of these leaded fuel grades havebeen referred to as low lead, or “LL”. One advantage of TEL is the verylow concentration needed. Other anti-knock agents must be used ingreater amounts than TEL, often reducing the energy content of thegasoline. However, TEL has been in the process of being phased out sincethe mid-1970s because of its neurotoxicity and its damaging effect oncatalytic converters. Most grades of avgas have historically containedTEL. This invention advantageously produces an unleaded grade ofgasoline which allows a range of piston engines to perform effectively.Therefore, in a preferred embodiment the inventive formulations andblends are unleaded, i.e., free of TEL. It is an object of the presentinvention to provide formulations that do not require deleterious octaneboosters, and which meet or exceed requirements for aviation gasoline.

A variety of fuel additives have been known and used in the art toincrease octane ratings, and thereby reduce knocking. Some embodimentsof the present invention utilize non-leaded combustion enhancingadditives individually or in combination with up to 6% by weight,esters, ethers, carbonates, C5-C7 cycloalkanes, or the use of triptaneand other known octane boosters.

Fuel components typically are not chemically pure, but instead maycontain other, non-deleterious fuel components. The term“non-deleterious fuel components” refers to components which are presentin a formulation other than as an intended component. Thus, selectedadditives such as mentioned above are not encompassed by this term.Instead, it refers more particularly to the fact that materials used incommercial embodiments of piston engine fuels may include constituents,e.g., hydrocarbons, which are present as contaminants to the componentsof primary interest. For example, an alkylate stream from a refinery maybe primarily composed of desired alkanes such as isobutane or isooctane,but may contain limited amounts of other hydrocarbons such as aromatichydrocarbons. As used herein, the term “substantially free of” refers tothe fact that the amount of such non-deleterious fuel components is lessthan about 5 wt %, preferably less than 2 wt % and more preferably lessthan 0.5 wt %, of the weight of the overall fuel formulation.

Thus, the fuel formulations may include a limited amount of aromatichydrocarbons, e.g., toluene, xylene, trimethylbenzenes, etc. Thesecompounds are frequently found in minor amounts in product streamsuseful for the present formulations. Moreover, in preparing fuels it isnot economical to use analytical grade or reagent grade chemicals, oreven technical grade chemicals, as the presence of other fuel-compatiblecomponents is not a concern, provided the resulting fuel formulationmeets ASTM and other applicable standards. Thus, the present inventioncontemplates the presence of such other fuel-compatible components inlimited amounts, e.g., less than 5 wt %, preferably less than 2 wt %,and more preferably less than 1 wt %.

All component percentages expressed herein refer to percentages byweight of the formulation, unless indicated otherwise. Given thesimilarity of the densities of the components of the present invention,it will be appreciated that the use of volume or weight percentages ofthe components in the ranges indicated provides comparable results.

The uses of the terms “a” and “an” and “the” and similar references inthe context of describing the invention (especially in the context ofthe following claims) are to be construed to cover both the singular andthe plural unless otherwise indicated herein or clearly contradicted bycontext.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

While the invention has been illustrated and described in detail in thedrawings and the foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. In addition, all references cited hereinare indicative of the level of skill in the art and are herebyincorporated by reference in their entirety.

1. A piston engine fuel formulation comprising: about 50 to about 75 wt% C₄-C₁₀ aliphatic hydrocarbons; about 20 to about 40 wt % ETBE;optionally up to about 3 wt % isobutane; optionally up to about 5 wt %C₆-C₁₂ aromatic hydrocarbons; and optionally up to about 250 ppmferrocene, said fuel formulation being free of lead-containingconstituents.
 2. The fuel formulation of claim 1 being substantiallyfree of C₆-C₁₂ aromatic hydrocarbons.
 3. The fuel formulation of claim 1and further comprising cumidine in an amount up to 5 wt %.
 4. The fuelformulation of claim 3 being substantially free of C₆-C₁₂ aromatichydrocarbons.
 5. The fuel formulation of claim 1 consisting essentiallyof: about 52 to about 80 wt % C₄-C₁₀ alkylates; about 20 to about 40 wt% ETBE; isobutane in an amount up to 3 wt %; optionally up to 5 wt %C₆-C₁₂ aromatic hydrocarbons; and ferrocene in an amount up to about 250ppm.
 6. The fuel formulation of claim 1 consisting essentially of: about57 to about 80 wt % C₄-C₁₀ alkylates; about 20 to about 40 wt % ETBE;isobutane in an amount up to 3 wt %; and ferrocene in an amount up toabout 250 ppm.
 7. A piston engine fuel formulation comprising: about 58to about 78 wt % isooctane; about 20 to about 40 wt % ETBE; about 2 wt %isobutane; and about 250 ppm ferrocene, said fuel formulation being freeof lead-containing constituents.
 8. The fuel formulation of claim 7consisting essentially of: about 58 to about 78 wt % isooctane; about 20to about 40 wt % ETBE; about 2 wt % isobutane; and about 250 ppmferrocene.
 9. The fuel formulation of claim 7 consisting of: about 58 toabout 78 wt % isooctane; about 20 to about 40 wt % ETBE; about 2 wt %isobutane; and about 250 ppm ferrocene.
 10. The fuel formulation ofclaim 7 comprising: about 58 wt % isooctane; about 40 wt % ETBE; about 2wt % isobutane; and about 250 ppm ferrocene, said fuel formulationhaving a MON of about 101.0.
 11. The fuel formulation of claim 7consisting essentially of: about 58 wt % isooctane; about 40 wt % ETBE;about 2 wt % isobutane; and about 250 ppm ferrocene.
 12. The fuelformulation of claim 7 consisting of: about 58 wt % isooctane; about 40wt % ETBE; about 2 wt % isobutane; and about 250 ppm ferrocene.
 13. Thefuel formulation of claim 7 and further comprising up to about 5 wt %C₆-C₁₂ aromatic hydrocarbons.
 14. A piston engine fuel formulationcomprising: about 50 to about 75 wt % C₄-C₁₀ alkylates; about 20 toabout 40 wt % ETBE; optionally up to about 3 wt % isobutane; andcumidine in an amount up to 5 wt %, said fuel formulation being free oflead-containing constituents.
 15. The piston engine fuel formulation ofclaim 14 comprising about 53 wt % isooctane; about 40 wt % ETBE; about 2wt % isobutane; and about 5 wt % cumidine.